code: def check_file(filename: str) -> List[LintMessage]:
      lint_messages = []
  
      with open(filename) as f:
          file = f.read()
          v = HasMainVisiter()
          cst.parse_module(file).visit(v)
          if not v.found:
              message = (
                  "Test files need to have a main block which either calls run_tests "
                  + "(to ensure that the tests are run during OSS CI) or raises an exception "
                  + "and added to the blocklist in test/run_test.py"
              )
              lint_messages.append(
                  LintMessage(
                      path=filename,
                      line=None,
                      char=None,
                      code=LINTER_CODE,
                      severity=LintSeverity.ERROR,
                      name="[no-main]",
                      original=None,
                      replacement=None,
                      description=message,
                  )
              )
      return lint_messages
  
file_path: pytorch/tools/linter/adapters/test_has_main_linter.py
imports: import argparse
  import json
  import multiprocessing
  from enum import Enum
  from typing import List, NamedTuple, Optional
  import libcst
  import libcst.matchers
label_desc: Use image node_id 1 for calling a global function with example usage: check_file(filename) and returns: lint_messages